> Another problem with posix_spawn() on glibc: it doesn't report errors to the parent process when run under QEMU user-space emulation and Windows Subsystem for Linux. This is because starting with commit [1] (glibc 2.25) posix_spawn()  relies on address space sharing semantics of clone(CLONE_VM) to report errors, but it's not implemented in QEMU and WSL, so clone(CLONE_VM) and vfork() behave like fork(). See also [2], [3].

Oh, you know a lot of stuff about vfork and posix_spawn, you are impressive :-)

Is sys.platform equal to 'linux' on WSL? Sorry, I don't know WSL. If it's equal, is it possible to explicitly exclude WSL in the subprocess test, _use_posix_spawn()?

For QEMU, I was very surprised that a full VM wouldn't implement vfork. So I tried Fedora Rawhide and I didn't notice the bug that you described.

---
$ cat test.c
#include <spawn.h>

int main(int argc, char *argv[], char *envp[]) {
    return posix_spawn(0, "non-existing", 0, 0, argv, envp);
}

$ gcc test.c
$ ./a.out
$ echo $?
2
---

In the VM, I get exit status 2 as expected.

You wrote:

---
$ aarch64-linux-gnu-gcc -static test.c
$ qemu-aarch64 ./a.out
$ echo $?
0
---

So the bug only affects "QEMU User Space". I never used that before. I understand that it's a bug in the glibc and in "QEMU User Emulation" which doesn't implement vfork "properly".


> There is no problem with musl (it doesn't rely on address space sharing).

I'm not interested to support musl at this point because I don't see any obvious way to detect that we are running musl nor get the musl version.

The history of this issue shows the posix_spawn() only works in some very specific cases, so we have to be careful and only opt-in for specific libc, on specific platform with specific libc version (read at runtime if possible).